Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field (B0 field) whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength. Transitions between these energy levels can be excited (spin resonance) by application of an electromagnetic alternating field (RF field, also referred to as B1 field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse), so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse.
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of one or more receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The MR signal data obtained via the RF coils corresponds to the spatial frequency domain and is called k-space data. A set of k-space data is converted to a MR image by means of Fourier transformation or other appropriate reconstruction algorithms.
Parallel acquisition techniques for accelerating MR acquisition are known in the art since many years. Methods in this category are SENSE (Sensitivity Encoding), SMASH (Simultaneous Acquisition of Spatial Harmonics), and GRAPPA (Generalized Auto-calibrating Partially Parallel Acquisition). SENSE, SMASH, and GRAPPA and other parallel acquisition techniques use undersampled k-space data acquisition obtained from multiple RF receiving coils in parallel. In these methods, the (complex) signal data from the multiple coils are combined with complex weightings in such a way as to suppress undersampling artefacts (aliasing) in the finally reconstructed MR images. This type of complex array signal combination is sometimes referred to as spatial filtering, and includes combining which is performed in the k-space domain (as in SMASH and GRAPPA) or in the image domain (as in SENSE), as well as methods which are hybrids.
Larkman et al. (Journal of Magnetic Resonance Imaging, 13, 313-317, 2001) propose to apply sensitivity encoding also in the slice direction in case of multi-slice imaging to increase scan efficiency. Breuer et al. (Magnetic Resonance in Medicine, 53, 684-691, 2005) improve this basic idea proposing an approach termed “controlled aliasing in parallel imaging results in higher acceleration” (CAIPIRINHA). This technique modifies the appearance of aliasing artefacts in each individual slice during the multi-slice acquisition improving the subsequent parallel image reconstruction procedure. Thus, CAIPIRINHA is a parallel multi-slice imaging technique which is more efficient compared to other multi-slice parallel imaging concepts that use only a pure post-processing approach. In CAIPIRINHA, multiple slices of arbitrary thickness and distance are excited simultaneously with the use of phase-modulated multi-slice RF pulses. The acquired MR signal data are simultaneously sampled, yielding superimposed slice images that appear shifted with respect to each other. The shift of the aliased slice images is controlled by the phase-modulation scheme of the RF pulses in accordance with the Fourier shift theorem. From phase-encoding step to phase-encoding step, the multi-slice RF pulses apply an individual phase shift to the MR signals of each slice. The numerical conditioning of the inverse reconstruction problem, separating the individual signal contributions of the involved slices, is improved by using this shift. CAIPIRINHA has the potential to improve the separation of the superimposed slice images also in cases in which the slices are rather close to each other such that the coil sensitivities of the used RF receiving coils do not differ dramatically in the individual slices to be imaged.
However, the conventional parallel multi-slice imaging approaches have limitations. When MR signals at multiple frequencies are simultaneously excited by a multi-slice (or multi-frequency) RF pulse, so-called side-band artefacts occur in the reconstructed images. These artefacts are caused by MR signals from regions excited unintentionally by one or more side-bands of the multi-slice RF pulse. The side-band frequencies may be higher order harmonics of the fundamental (main-band) frequency of the respective RF pulse. Such side-bands of the multi-slice RF pulse are unavoidable in practice due to hardware constraints of the used MR apparatus, e.g. non-linearity or the RF amplifier. The characteristics of the side-band artefact depends on the individual load of the RF coil arrangement, the B1 distribution within the examination volume, and the fundamental frequencies involved in the multi-band excitation.